The present invention relates to a level shifting circuit, and more particularly to a level shifting circuit using an operational amplifier.
A prior art level shifting circuit using an operational amplifier is shown in FIG. 1 in which an input voltage V.sub.i to be level shifted is applied through a resistor R.sub.1 to a non-inverting input of the operational amplifier 1 while applying thereto a level shifting voltage E through a resistor R.sub.2. Negative feedback using an output voltage V.sub.0 of the operational amplifier 1 is provided through a negative feedback circuit including resistors R.sub.3 and R.sub.4.
Provided that the operational amplifier 1 is substantially ideal, that is if its gain and its input impedance are essentially infinite, and the resistances of the resistors R.sub.1 through R.sub.4 are all equal to R, then the non-inverting and inverting input voltages V+ and V- can be respectively expressed as follows: EQU V+=(E+V.sub.i)/2, and EQU V-=V.sub.0 /2. (1)
Since +V is approximately equal to V-, the following equation can be obtained from the above equations (1). EQU V.sub.0 =V.sub.i +E. (2)
As is apparent from the above, the output voltage V.sub.0 of the operational amplifier 1 is level shifted by the voltage E with respect to the input voltage V.sub.i.
In order to satisfy the above equation (2), it is necessary that the impedance of a signal source be zero or extremely small in comparison with the resistance value R of the resistors.
Due to the specific relation between the input voltage V.sub.i and the level shifting voltage E, a current may flow back to the input side of the operational amplifier 1 from the level shifting power source E. Such a condition should be avoided, depending upon the nature of the signal source. Accordingly, in the case where the backflow of the current must be prevented or the impedance of the signal source is not neglegible, it is necessary to provide a buffer circuit.
Moreover, there is a requirement that the level shifting voltage E be varied in the circuit shown in FIG. 1. In order to comply with such a requirement, however, a disadvantage is introduced in that the construction of the level shifting voltage source becomes complicated.